1. Field of the Invention
The present invention is directed to a force sensor for making a direct force measurement in the force flux, i.e., in the force field produced by a force to be measured. A direct force measurement means that the directly charged with the force to be measured. Usually disposed to oppose or yield to the force to be measured, the force sensor thereby transmits the force to be measured and simultaneously measures it.
2. Description of the Art
The actuation of brakes, particulary in the motor vehicle field, currently ensues hydraulically. For various reasons, a switch to purely electrically actuated via an electric motor with gearing. This can occur, for example, on the basis of the motion of a spindle running in an electric motor.
A constant measurement of braking force is required for safety reasons as well as for controlling the braking procedure. This should ensue directly at the brake shoes insofar as possible. Considerable temperature differences, however, prevail in this region. The temperature range to be taken into consideration at a caliper extends from approximately -40.degree. C. up to approximately 350.degree. C. The range of force measurement extends up to a few tons. Additionally, of course, a high precision should be assured, for example a measuring reliability with less than 1% deviation. Additionally, there is a need to accommodate very rapid temperature changes which occur during normal operation of a motor vehicle brake. Enormous impact stresses perpendicular to the motor vehicle axle are also present. The force sensor thus should have a permanent load alternation endurance as well as an absolute insensitivity to moisture. Insensitivity to electromagnetic emission is likewise meaningful. Other complicating factors are that only a small space is available in the region of the motor vehicle brake, and the system costs should be optimally low.
Known force sensors are, for example, piezosensors or strain gauge time sensors. The latter represent an indirect measuring method, whereby the sensor does not lie in the force flux. Neither of these systems even approximately satisfies the aforementioned extreme demands. Other systems, for example, are capacitive pressure and silicon-based force sensors. These, however, do not satisfy the extreme operating conditions or the cost demands.